The present invention relates generally to video disc players and more particularly to video disc players having radio frequency interference reduction apparatus.
In certain types of video disc players, such as, for example, capacitive pickup video disc players, the signal pickup circuitry includes an oscillator operating in conjunction with a tuned circuit and peak detector. An example of such pickup circuitry may be found in U.S. Pat. No. 4,080,625 issued to H. Kawamoto et al. on Mar. 21, 1978. The oscillator frequency in such systems is generally set at a value falling within the ISM (industrial, scientific, medical) allocation band, for example, 915 MHz. The power output of this oscillator at 915 MHz is usually low and does not generally represent a significant problem in terms of r.f. radiation outward. However, there are certain types of equipment which operate at high power levels in the ISM band, such as radars for example, and, if a number of conditions are met (such as frequency, power level, directivity and location, etc.) these outside sources of radiation can have an adverse effect on the playback operation of the video disc system.
This undesirable mode of operation appears to manifest itself in the following way. When an outside source is radiating at levels which are higher than those anticipated by the pickup circuit detector in normal player operation, the extraneous detected output signal is supplied to an FM detector which locks onto the extraneous signal. The extraneous signal is then demodulated and passed through the player signal processing circuits and displayed on the television receiver. If the extraneous signal is a pulse of short duration-say two microseconds, then the demodulated version may appear on the output television display as a short interval of black or white on a horizontal line. This results from the fact that in FM systems of the type under consideration detection of high carrier frequencies is decoded as tending toward "white" level signals and detection of low carrier frequencies is detected as tending toward "black" level signals. These short duration interference signals may be noticeable to the discerning eye.
When the external source is of greater duration, say 125 microseconds, the effect is much more noticeable in the output display. Since one television horizontal line in the NTSC format is about 63 microseconds long, a condition may arise wherein two lines of the display will be white or black. Now if the outside source is a periodic type source, then two lines of the display will show up incorrectly on a corresponding periodic basis. This is noticeable even more than the previously mentioned situation.
There are various combinations of outside source power levels and durations which may produce other undesirable results in the final display such as a combination of light and dark spots on one or more horizontal lines of the display and even picture rolling if the FM demodulator falsely locks on to an extraneous signal when the vertical synchronization signal should normally be detected. In respect of all of the above identified display problems, it is desirable to provide a means to at least reduce the effects of the interference to a level which most observers would not even notice. Again, it should be noted that the problems mentioned arise only under a certain set of conditions.
In a patent application filed on Nov. 7, 1980 in the names of F. B. Lang and J. K. Clemens, now U.S. Pat. No. 4,327,432 a solution to the above situation is presented.
The Lang et al. invention provides a means for reducing the effects of outside ratio frequency interference (RFI) in the operation of a video disc player of the type which includes a pickup circit means coupled to an FM demodulator. The FM demodulator passes the FM detected signal to a defect compensation means which includes a defect switch and a delay line. The FM demodulator is also coupled to a defect detector which senses normal defects, that is, when the FM carrier deviation is beyond certain deviation limits. The normal operation of the defect compensation means is to recirculate and reprocess a previous line of demodulated information when the defect detector operates the defect switch. In accordance with the Lang et al. invention, a radio frequency interference reduction circuit is connected between the output of the pickup circuit means and the defect switch. The RFI reduction circuit, in one embodiment, comprises an amplifier, coupled to a detector, which in turn is connected to two conduction paths. The two conduction paths are connected to a differential signal comparator circuit. One of the two paths is a wideband path and the other path includes a low pass filter which does not pass abrupt changes in the signal provided at the output of the pickup circuit. Either one of the paths may have a d.c. offset voltage applied thereto. The net effect is that the differential input to the comparator provides an output in response to rapid amplitude variations in the output signal of the pickup circuit. When such a situation is detected, the comparator provides a signal to the defect switch, through an OR function circuit, to cause recirculation of a previously detected line of information for the duration of the detected defect. The OR circuit is used so that the recirculation may be accomplished for either the abnormal carrier deviations caused by the disc defects or the abnormal occurrence of a detected extraneous outside signal.
Another patent application was filed on Nov. 7, 1980 in the names of C. B. Dieterich and F. B. Lang, now U.S. Pat. No. 4,327,431 wherein a system of the Lang et al. type was presented with the addition of a variable offset voltage in one of the two paths.
In a patent application filed for J. K. Clemens Ser. No. 208,984, filed Nov. 21, 1980, now U.S. Pat. No. 4,361,881, an approach is taken wherein the signal from the pickup means is subtractively combined with a limited version of the same signal to enhance the ability to detect the extraneous signal. In still another patent application filed in the name of J. J. Gibson Ser. No. 208,982 filed Nov. 21, 1980, now U.S. Pat. No. 4,364,119, the signal from the pickup means is multiplied with the signal from the limiter as another approach to detection.